The long-term goals of the project proposed are to characterize the role of hepatocytes in the resolution of systemic bacterial infections and to delineate the factors that mediate hepatocyte resistance to infection. Although most of the bacteria that enter the bloodstream are removed by the liver, these organisms are rarely cultured from liver biopsies. The mechanisms that underlie clearance of the blood and sterilization of the liver remain to be elucidated. Using listeriosis in mice as an experimental model, we found that the bulk of the bacteria injected into mice i.v. was taken up by hepatocytes. While the organisms proliferated exponentially within the hepatocytes of nonimmune mice, they were eliminated rapidly from the hepatocyte -population of immune animals. Both T lymphocytes and natural killer cells have been implicated in host defenses to Listeria monocytogenes. A combination of in vivo cell depletion and adoptive-immunization techniques will be used to identify the immune cell population(s) that provides hepatocytes protective immunity to Listeria. The mechanisms by which the active cell population confers resistance will be examined. Modulation of cytokine production in the liver by immune cell populations will be assessed by Northern blotting analysis, in situ hybridization and immunohistochemical techniques. The effect of recombinant cytokines on the replication of Listeria within hepatocytes in vivo and in vitro will be determined. To ascertain the basis of cytokine activity, the effect of cytokines on the uptake of Listeria and the generation of antimicrobial metabolites, e.g. reactive oxygen intermediates, will be assessed. The capacities of immune T lymphocyte (subsets) and natural killer cells to lyse infected hepatocytes in vitro will be analyzed using a conventional chromium release assay. The study proposed in this application is the first to examine specifically the role of hepatocytes in the resolution of systemic bacterial infections. The mechanisms operative in the immune response to Listeria have proven to be important in the response to a variety of pathogenic microorganisms. Consequently, the results of this study should provide significant insight into the factors that affect the elimination of the many organisms capable of causing systemic, as well as hepatic, infections.